1. Field of the Invention
The present invention relates to an excitation system for a synchronous electric power generator system, and in particular to an excitation system for a synchronous electric power generator system having by-pass capability.
2. Background Information
Synchronous electric power generators systems are well-known in the art and are used to produce alternating current output voltages, such as single phase or three phase voltage outputs. An example of a prior art synchronous electric power generator system is shown in FIG. 1 at numeral 5. As shown in FIG. 1, a typical synchronous electric power generator system includes an AC generator 10, coupled to power system 12, that has a stator 15 and a rotor 20 having a field winding excited by a field voltage provided by an exciter 25, which commonly includes a plurality of SCR bridges, such as is described in U.S. Pat. No. 6,046,917, entitled “Controlled Rectifier Bridge and System,” owned by the assignee of the present invention and incorporated herein by reference. Current in the rotor 20 creates a rotating magnetic field which induces current in the stator 15 to produce an output voltage at the terminals of the stator 15. The amount of field voltage provided by the exciter 25 controls the generator field magnetic strength. As the main generator field strength is controlled, so is the generator output voltage induced in the generator stator 15. In the typical case, the exciter 25 is under the control of an automatic voltage regulator 30, known in the art, which is connected to the output voltage terminals of the stator 15 by system voltage transducer 35. The automatic voltage regulator 30 controls the voltage applied to the exciter 25 which adjusts the generator output voltage by controlling the strength of the main generator field. Many different techniques are used to sample or sense the output voltage and provide feedback to the automatic voltage regulator 30, one example of which includes detecting the peak of every other half cycle of the output voltage, comparing it with a desired reference value, and then adjusting the exciter voltage up or down as required to correct the generator output voltage. This type of feedback control system is known in the art as an outer loop voltage regulator.
It is also known in the art that what is known as an inner loop voltage regulator system may be used to linearize the firing circuit control for the exciter 25. When an inner loop voltage regulator system is employed in a synchronous electric power generator system 5, the system further includes automatic voltage regulator 40 as shown in FIG. 1. Automatic voltage regulator 40 is connected to the output of the exciter 25 by a field voltage transducer 45 in order to receive the field voltage as an input. The reference for the automatic voltage regulator 40 is the output of the automatic voltage regulator 30.
The problem with the synchronous electric power generator system 5 shown in FIG. 1 is that, if field voltage transducer 45 fails, then the outer loop voltage regulator system is no longer functional. Thus, there is a need for a system that will enable continued operation of a synchronous electric power generator system in the event that the inner loop voltage regulator system becomes non-functional, such as from the failure of a field voltage transducer like field voltage transducer 45.